Microelectronics, microsensors, and microelectromechanical systems (MEMS) typically utilize energy sources located off-chip. Integrating microscale energy storage on-chip with microdevices is essential for achieving autonomous devices. Electrical energy for microdevices can be provided by either capacitors or batteries. Capacitors can discharge very quickly, but inherently contain very little energy. Traditional batteries contain large amounts of energy, but cannot discharge quickly. Other power sources, such as fuel cells, are practical for larger systems, but are not easily miniaturized.
Batteries are limited by their maximum power density/discharge rate because of slow kinetics related to ion and electron transport. Reducing the characteristic ion and electron diffusion lengths within the active battery material has proven to be successful in increasing power densities and discharge rates; however, this has also resulted in a substantial decrease in energy density. Miniature batteries have been developed to power cm2 sized devices and microelectronics, but they have not seen widespread adoption due to limits in their energy and power capabilities. Thin-film lithium ion batteries, for example, have high power densities due to thin active material layers (<1 μm), but the total power and energy provided is generally not sufficient to meet the demands of micro devices due to the two-dimensional architecture inherent to thin films. Building into the third dimension—e.g., making thicker active material layers—can boost the energy density; however, electron and ion diffusion lengths concomitantly increase, thereby reducing power density.